Molecular Modelling of Humic Substances, Soil Organic Matter and Soil Particles: Potential and Limits

Abstract

The combination of pyrolysis-field ionization mass spectrometry (Py-FIMS) and Curie-point pyrolysisgas chromatography/mass spectrometry (Py-GC/MS) together with a wide variety of analytical (chemical, thermal, spectroscopic) techniques has been utilized in an integrated approach to model molecular structures for humic acids (HA), fulvic acids (FA), total humic substance (THS), dissolved organic matter (DOM), soil organic matter (SOM) and soil particles. Personal computer-based molecular modelling of humic-xenobiotic complexes and computational chemistry (molecular-mechanics calculations; geometry optimization; determination of binding sites; transport-, energy-balances, quantitative structure-activity relationships) are reported. Examples for atrazine-, hydroxyatrazine-, DDT-, mecoprop-, metribuzin-, pentachlorophenol-, trinitrotoluene-, and dioctylphthalate-interactions with THS, DOM, SOM and soil particles are given and illustrate the processes of trapping and binding anthropogenic substances in soil and water (Schulten et al., 1998). Hypothetical structural models for humic substances such as HA monomer (Schulten et al., 1991) Schulten and Schnitzer, 1993) and oligomeric humic acids (Schulten, 1996) were proposed in an integrated approach using a wide variety of analytical methods. The HA model is important because humic substances constitute 70-80% of soil organic matter (SOM) and all chemical reactions of the latter (large surface, voids, high adsorption capacity, good metal complexer, good medium for microorganisms, can store nutrients and especially water) can be explained on the basis of HAs. From the analytical aspect, main emphasis was put on pyrolysis-mass spectrometry and pyrolysis-gas chromatography/mass spectrometry. Modem personal computer-based techniques of molecular modelling (Hypercube | Inc., 1115 NW 4th Street, Gainesville, Florida, 32601-4256, U.S.A.) were employed to develop three-dimensional structures of humic acids (Schulten, 1995; Schulten and Schnitzer, 1993) and organomineral soil complexes (Schulten, 1996). In particular trapping and bonding of biological (peptides, carbohydrates) and anthropogenic substances such as pesticides and plastizisers in the voids of the geometrically optimized structures were investigated. Basis of the presented work were improved structural concepts of SOM and soils (Schulten and Schnitzer, 1997) and the development to model humic xenobiotic complexes in water (Schulten et al, 1998). In a recent survey on the role and structure of organic nitrogen in soils, the combination of analytical pyrolysis proved to be particularly powerful when the results of sophisticated analytical methods could be supported and by computer modelling (Schulten et al., 1998). Despite an inspiring dispute on the value or nonsense of three-dimensional models of humic substances, in the meantime excellent textbooks on environmental soil chemistry cite and display these model structures.